As society becomes increasingly mobile and technologically advanced, batteries are playing a more important role. In particular, the need for batteries is growing especially rapidly due to the increasing use of mobile telephones, portable computers, camcorders, hybrid electric vehicles, and distributed power applications (including solar and remote).
For cost-effectiveness and for environmental protection, this demand for batteries has particularly increased the demand for rechargeable (or secondary) batteries. Consumers demand rechargeable batteries characterized by long cycle life, rapid charge capacity, high energy density, and small size and weight for powering their portable electronic devices.
Rechargeable batteries based on lithium metal anodes provide one approach to satisfying this demand due to their high energy density. Lithium ion batteries generally have an energy density greater than that of metal hydride cells and that of nickel/cadmium cells. The low internal resistance of a lithium cell that has a liquid electrolyte generally provides a solution with a higher power density and a greater cell life (i.e. more charge/discharge cycles).
It is desirable, and sometimes necessary, for a battery package to be sealed. A sealed package inhibits external contaminants from reacting with the cell and prevents the cell components from leaking out of the package. In the case of a lithium/lithium ion/lithium ion polymer cell in particular, an insufficient seal may result in the lithium/lithium electrolyte salt reacting with moisture in ambient air that enters the cell. The reaction can produce a passivation film on the lithium cathode surface which increases the internal resistance of the cell, thereby reducing cell performance. The reaction can also consume the lithium salt, thereby reducing cell performance. In the case of leakage of a liquid cell, in addition to not wanting the electrolyte to leak and thereby damage a device or harm a person, there are also is a risk of ignition of the liquid electrolyte solution.
Although it is desirable to seal a battery for the reasons described above, a sealed battery brings with it another set of problems. The charging and discharging (or sometimes overcharging, overdischarging, or short circuiting as the case may be) of a lithium cell often results in the generation of gas that causes the internal pressure of the cell to rise. The increase of internal pressure may cause the cell to deform and thereby deteriorate cell performance or perhaps even ultimately cause the seal to rupture and thereby terminally damage the cell. It is further desirable to use flexible pouch packaging. However, heat-sealed flexible pouch packaging allows a finite amount of diffusion of moisture and oxygen into the cell.
FIG. 1A is a cross-sectional diagram of a portion of a conventional flat battery 100. The battery 100 includes multiple layers of anodes 102, multiple layers of cathodes 104, and multiple layers of electrolyte 106 sealed in a flexible enclosure 108. The affects of the generation of gas are not shown in the battery 100 of FIG. 1A.
FIG. 1B is a cross-sectional diagram of a portion of the battery 100 shown in FIG. 1A that has experienced an increase in internal pressure and resulting loosening or deformation due to migration of gas through the seal or the generation of gas by chemical or electrochemical reaction. The gas 110 is shown in a layer of the electrolyte 106-A. The pressure caused by the gas 110 causes the electrolyte 106-A, adjacent electrodes 102-A, 104-A, electrolyte 106-B, and the enclosure 108 to deform. In addition to the increase of pressure within the enclosure 108 potentially causing the enclosure 108 to rupture and terminally damage the battery 100, the deformation caused by the generated gas 110 will reduce the performance of the battery 100 due to the loss of proper orientation/spacing between the anode, cathode, and electrolyte layers.
Conventional solutions to alleviate the affects of increased gas pressure include designing the battery package with a weak point or a vent that will break open in response to an increase of pressure beyond a predetermined threshold. The battery is thereby prevented from bursting by releasing the internal pressure to the ambient surroundings through the breakage. Although this solution may result in the cell not bursting, the cell may be terminally damaged and may leak through the breakage.
The generation of gas is characteristic of rechargeable lithium/lithium ion/lithium ion polymer cells/batteries. Gas is usually formed during the first charge cycle (often called the formation cycle) and to a lesser degree for many cycles thereafter. To compensate for this generated gas, flat cells are generally reopened and degassed after the first charge cycle. Wound cells in metal cans are generally not degassed after formation as the metal case is less affected by the resultant pressure. Even after degassing, on continued cycling and/or standing, a certain amount of gas (either from electrochemical reaction or migration of moisture and/or oxygen through the seals, or chemical reaction with said moisture or oxygen) will accumulate within the package.
In addition to the seal, it is also desirable to keep cell electrodes in close proximity to each other for good performance. In polymer cells, this is generally achieved by laminating the electrodes to the separator to form a self-adhering composite. Liquid cells are generally wound wherein the winding holds the layers together. Liquid cells may also be fabricated with flat electrodes that are clamped between two plates to hold the layers together. Lithium metal cells can be fabricated by any of the above techniques.
However, rigid restraints such as clamping arrangements are inflexible with regard to manufacturing variations. For example, FIG. 2 is a cross-sectional diagram of a portion of a conventional battery 200. The coatings 202 and separators 204 are clamped between two plates 206. In this case, the coatings 202 have a non-uniform thickness due to manufacturing variations. The inflexibility of the clamping arrangement causes a greater pressure at the thick spots 208 in the coatings 202 which causes a corresponding variation in thickness of the separator 204. Variations in separator thickness result in corresponding changes in ionic resistance, thereby reducing performance.
Battery packaging requirements and the difficulty in maintaining the seal have resulted in battery manufacturers compromising between battery performance and packaging flexibility. On the one hand, a battery with a liquid or gel electrolyte generally has a higher power density, greater cell life, and costs less. However, such batteries are manufactured with a rigid and substantial package to hold the electrodes together, usually by winding. On the other hand, batteries with a polymer-based electrolyte can be manufactured in a thin flat format using a flexible foil package.
There is a need for an improved battery package system that alleviates the performance-deteriorating affects of increased pressure without permanently damaging the battery to allow the manufacture of a high capacity battery in a flexible package.